This invention relates to a method and structure for providing support for walls and ceilings of underground tunnels by inserting bolts into holes drilled in the walls and ceilings.
Artificial roof and wall support in underground mines has been utilized for many years to protect men and equipment from cave-ins or other mine failures, and to maintain a mine open and accessible over extended periods of time. One method of providing such artificial support is by way of roof bolts placed in holes drilled in the mine walls and ceilings. The use of roof bolts has proven to be effective in increasing a mine's structural stability and in preventing accidents and injuries caused by cave-ins.
Mining roof bolts in current use generally fall into three categories identified as point-anchor bolts, full contact bolts, and combination bolts (there may be other categories of mining roof bolts not mentioned). A point-anchor bolt is one which is anchored in place in a hole in the mine roof or wall by some mechanical anchor on the end of the bolt away from the bolt head. As the bolt is tightened ("torqued"), the mine strata (layers of rock) are compressed together between the bolt head and mechanical anchor to thereby stabilize the strata. Point-anchor bolts are relative inexpensive, and therefore are quite popular, but they have a number of disadvantages including the tendency of the bolt to creep or slip with the passage of time resulting in a loss of needed roof tension and thus support. This gives rise to a need to retorque the bolt--a dangerous and expensive procedure. Another disadvantage is that the rock near the point of anchoring is oftentimes caused to fracture. And furthermore, in wet mine environments, corrosion or disintegration of the rock and roof bolt result. Finally, the point anchor bolt is not suitable for use in soft rock such as coal, shale, etc.
The full contact bolt provides for anchoring the bolt in the bore hole along the substantially full length of the bolt. This type of configuration gives added strength and is fairly resistent to creep or slippage. Two examples of full contact bolts are the resin bolt and the Scott split set. The resin bolt consists of resin cartridges which are placed in the hole and a bolt similar to a length of rebar which is inserted in the hole and then spun to break the resin cartridges and allow the resin to fill the hole and harden. The resin bolt, although providing full length anchorage, is fairly costly and difficult to install. The Scott split set (this may be a trademark of Ingersol Rand) consists of a hollow, compressible steel tube having a slit formed along its length along one side. The tube is compressed and driven with considerable force into a hole. The tube expands in the hole and secures itself against the walls of the hole. Like the resin bolt, the split set distributes the anchoring force along the length of the tube. Disadvantages of the Scott Split Set include difficulty of installation, tendency to fracture the rock as the bolt is driven into place, and limited effectiveness in irregular holes and in stratified layers of various hardnesses such as is common in mines. The reason for this limited effectiveness is that the tube, as it expands, does not compliantly conform to hole shape. The hard layers of rock prevent the steel tube from expanding and exerting a uniform pressure on the soft rock and as a result the overall load capacity is reduced.
Prior art patents which illustrate various roof bolts structures and also various anchoring devices generally include L. Triplett, U.S. Pat. Nos. 3,504,498, L. Triplett, 3,492,909, O. H. Varga, 4,108,043, J. C. Lang, 2,950,602, J. E. Munn, 3,349,567, I. Cohen, 3,017,800, and R. S. Peirse, 2,203,178. The arrangements disclosed in these references include one or more of the disadvantages of the roof bolts discussed above or are simply not applicable to providing the desired roof and wall support needed in underground mining.